Or is it just the SPL drops off due to the poor horizontal dispersion associated with a horizontal speaker. I have never noticed the sound actually changing just the volume drop. I have used my SPL meter and measured a 1-2 db drop off and once I adjusted the volume back up the sound was the same again.
Try flipping the centre, then stand on your couch, then sit, then kneel down and listen to all 3 locations.
The sound changes with axis position.
Flipping horizontal to vertical puts those drivers in a different height orientation.
The sound is very different.
